A solar cell is a photovoltaic device comprising p-type and n-type materials and having a p-n junction at the interface between them for the conversion of solar energy directly to electrical energy. A solar battery usually consists of a plurality of solar cells connected in series and parallel to each other in a matrix fashion where a large power output is required. Hitherto, commercial solar batteries comprising a p-n, homo-, or hetero-junction of monocrystalline or polycrystalline Si, GaAs, CdS, CdTe or the like are available as well as those comprising a p-type, an i-type and an n-type layer of amorphous silicon each deposited in series and forming a p-i-n junction. Moreover, it has been suggested that a solar battery comprising a plurality of solar cells, each of which has a p-type, an i-type and an n-type amorphous silicon layer each deposited in series, may produce a high output voltage.
In the disclosure herein described, the following terms are to be construed as having the following meanings. The term "heterojunction" (HJ) is a junction formed between semiconductor regions made of different materials. The term "junction" is a homojunction made of the same materials and is often used as including a heterojunction. The "p-n junction" is a region of transition between p-type and n-type semiconductor regions. The term "p-i-n junction" is a region of successive transition between a p-type and an i-type semiconductor region and between the i-type and an n-type semiconductor region. In the field of solar cells, it is only used for solar cells made from amorphous material and plays the same role as the p-n junction in a solar cell made of crystalline material for the photogeneration of carriers. The term "amorphous p-n junction" is a region of transition formed between p-type and n-type amorphous semiconductor regions, and provides an ohmic contact between the two regions. The term "unitary cell" is sometimes used to name a solar cell which constitutes a tandem junction solar battery. A crystalline solar cell is a solar cell wherein a p-n junction is formed from crystalline regions. An amorphous solar cell is a solar cell wherein a p-i-n junction is formed from amorphous materials. A heterojunction (HJ) solar cell is a solar cell comprising a crystalline region and an amorphous region and having a p-n heterojunction between them. The adjective "amorphous" is often used as including a microcrystalline structure because a microcrystalline semiconductor may be substituted for an amorphous semiconductor in many cases.
Solar batteries must be economical for them to be widely utilized in practice. It has generally been considered that a thin-film process may lower the manufacturing cost of a solar battery. In this respect, a manufacturing process using amorphous silicon is considered desirable because it may reduce the thickness of an amorphous semiconductor region down to the order of 0.5 micrometer in contrast to about 300 micrometers in thickness of a monocrystalline silicon region which is generally employed in a crystalline or HJ solar cell. It has been expected that a thin-film process may produce a large-area device having a large light-receiving area.
The efficiency to convert solar energy into electric energy is closely related to the manufacturing cost of a photovoltaic device. An amorphous silicon solar battery has a disadvantage of having lower conversion efficiency than that of a monocrystalline silicon solar battery, though the amorphous silicon solar battery is expected to have a lower manufacturing cost when a thin film process is used to make it. FIG. 1 shows a graph of the collection efficiency .eta. in relative units of an amorphous silicon (a-Si) solar cell and that of a monocrystalline (mono-Si) solar cell plotted against the photon wavelength .lambda. in micrometers. The conversion efficiency is related to the area that the collection efficiency curve subtends. As shown in FIG. 1, the mono-Si curve extends to a relatively long wavelength, whereas the a-Si curve is limited to a shorter wavelength. Therefore, the conversion efficiency of commercial amorphous silicon solar batteries is 7-8%, which is lower than that of a monocrystalline silicon solar battery because the former may not utilize solar energy as effectively as the latter does.
In order to improve the conversion efficiency of a solar battery, a multi-layered solar battery has been studied which usually comprises semiconductor regions having different energy gaps. The region with the largest energy gap is positioned for it to absorb the incident light first, and the other regions are positioned further in series with the incident light in order of decreasing energy gap in order that the lower energy photons may penetrate deeper. Each region has its own peak value of collection efficiency at a predetermined range of wavelength in the spectrum of solar light. This type of a device, called a tandem junction solar battery, may raise the conversion efficiency because it utilizes the incident light more effectively than a solar cell made of one material does.
As for the manufacturing process, it is desirable to adopt a low temperature process. In crystalline solar cells, a p-n junction is generally prepared by using junction formation methods including a high temperature process such as a thermal diffusion process or an annealing process after an ion implantation. The former is a process wherein impurities are diffused thermally into a crystalline semiconductor which is kept at a prescribed temperature as high as about 1000.degree. C. in an environment containing dopant impurities. The latter is a process wherein ionized dopant impurities are accelerated under a high electric field so as to be implanted into a crystalline semiconductor. In the ion implantation process, it is necessary to anneal the implanted semiconductor at a temperature as high as about 1000.degree. C. because the crystal structure is largely disordered when once the ions have been implanted. However, a high temperature process requires a complex set-up and careful control in operation, and this increases the manufacturing cost of a solar battery. In addition, it lowers the conversion efficiency of the solar battery because undesirable impurities migrate into the crystalline semiconductor from surroundings.
With respect to an amorphous solar cell, the width of the energy gap which affects the collection efficiency may be adjusted by selecting the kind and/or the mixing ratio of the gases supplied during the manufacturing process thereof. Hydrogenated amorphous silicon carbide, SiC:H, and/or hydrogenated amorphous silicon nitride, SiN:H, have been used for shorter wavelengths, while hydrogenated amorphous silicon germanide SiGe:H and/or hydrogenated amorphous silicon stannide SiSn:H have been used for longer wavelengths. However, though a hydrogenated amorphous semiconductor (e.g., a-SiGe:H or a-SiSn:H) having a narrow energy gap can absorb light of longer wavelength, a multi-layered amorphous solar battery comprising various regions thereof cannot satisfactorily convert the light of longer wavelength to photocurrent because of inferior photovoltaic properties of an amorphous solar cell. On the contrary, crystalline semiconductors (e.g., Si, GaAs, or Ge) having a narrow energy gap can utilize satisfactorily the light of longer wavelength.
The reflection of the light incident on the surface of a solar cell is also important for its performance. An anti-reflecting transparent layer having a suitable refractive index and thickness has been applied on the surface of the outermost semiconductor region so as to effectively minimize the reflection.
In tandem junction solar batteries, it is necessary for the interface between unitary cells to provide a contact which does not bring about an electrical and optical loss in performance. A heavily-doped tunnel junction or a transparent conductive layer has been inserted to provide an ohmic contact. For example, in U.S. Pat. No. 4,292,461, an indium-tin-oxide transparent conductive layer is placed between a p-type crystalline silicon region and an n-type amorphous silicon region.
It should be noted, however, that the addition of such an extra material as indium-tin-oxide is liable to deteriorate an amorphous solar cell because of undesirable impurities such as indium atoms or oxygen atoms migrating into the amorphous region during the deposition. Since the unit cell having the lowest performance among the several unit cells comprising a tandem junction solar battery restricts the performance of the tandem junction solar battery, each of the units cells must have the good performance so as to obtain high conversion efficiency. Therefore, the addition of such an extra layer as indium-tin-oxide is undesirable because it brings about the deterioration of an amorphous solar cell resulting in the lowering of the performance of a tandem junction solar battery. It should also be noted that the addition of such an extra material as a transparent conductive layer may increase the manufacturing cost since it is usually deposited by using such an expensive process as an electron beam deposition process or a sputtering process and since the adoption of such an extra process interrupts the continuity of the manufacturing process.